The increase in worldwide industrialization has generated concern regarding pollution created by combustion processes. Particularly, emissions from vehicles or other distributed sources are of concern. New environmental regulations are driving NOX (a mixture of NO and NO2 of varying ratio) emissions from diesel fueled vehicles to increasingly lower levels, with the most challenging of these being the 2010 EPA Tier 2 diesel tailpipe standards. To meet these, engine manufacturers have been developing new diesel after-treatment technologies including selective catalyst reduction (SCR) systems and lean NOX traps (LNT). See for example: T. Johnson, 2008 SAE International Proceedings, 2008-01-0069 (2008). These systems require multiple NOX sensors to monitor performance and satisfy on-board diagnostics requirements for tailpipe emissions. Point of generation abatement technologies have been developed for NOX, among other pollutants, but these solutions can reduce fuel efficiency if they are applied without closed loop control. Further, some of the proposed solutions can be polluting (e.g. selective catalytic reduction systems for NOX can release ammonia into the atmosphere) if improperly controlled. Control of these abatement technologies requires the development of compact, sensitive sensors for NOX and other pollutants in oxygen-containing (lean-burn) exhaust streams.
Sensors that have been proposed to date cannot meet the requirements of the applications. The great majority of NOX detectors rely on the potentiometric or amperometric measurement of oxygen partial pressure (from the decomposition of NO2 molecules to NO and NO to N2 and O2) to determine NOX concentration. This requires that the device be constructed with reference electrodes or reference pumping circuits to separate the NOX concentration from the background oxygen concentration.
Electrochemical sensors offer a means of measuring gas constituents in an analyte stream using a small, low power device. A number of electrochemical sensor approaches have been reported in the past. See for examples: J. W. Fergus, Sensors and Actuators B121, 652-663 (2007); W. Gopel, et al., Solid State Ionics 136-137, 519-531 (2000); and S. Zhuiykov, et al., Sensors and Actuators B 121, 639-651 (2007). These approaches range from potentiometric mixed potential sensors to impedance-based sensors to amperometric sensors. Most of these approaches employ a ceramic electrolyte material as one component of the device, with electrode materials that provide sensitivity to a gas species of interest. A broad scope of materials have been evaluated as the sensing and reference electrodes in these designs, including precious and base metals, as well as cermets, and both simple and complex oxides. The electrolyte selection has been much narrower, focusing principally on yttrium-stabilized zirconia and a minority of examples of NASICON electrolytes. None of these approaches meets all of the key requirements of the diesel exhaust application.
Mixed potential designs rely on the different kinetics of reaction to occur at the sensing and reference electrodes. For the example of NOX detection, two reactions are of interest:the reduction of NO2 to NO: NO2→½O2+NO; and/or  (1)the reverse reaction of oxidation of NO to NO2: NO+½O2→NO2.  (2)These reactions occur at different rates over different electrode materials. The local liberation or consumption of molecular oxygen changes the oxygen partial pressure at the sensing electrode, and results in a change in the electromotive force (EMF) generated in contrast to the reference electrode. Reference electrodes are selected to be inert to these reactions but active for O2 reduction (such as Au or Pt). Examples of sensing electrodes for mixed potential sensors include simple oxides such as WO3, NiO, ZnO, Cr2O3, V2O5 or mixed oxides such as spinels composed of di- and trivalent transition metals, or lanthanide ferrite or chromite-based perovskites. Because the reference electrode compensates for oxygen that may be present in the gas stream, the EMF between the sensing and reference electrodes can be correlated directly with the concentration of NO or NO2 present.
Drawbacks to the mixed potential approach include the interference of other gas species with the sensing and reference electrodes. Reducing gases present in the gas stream, such as hydrocarbons and CO, will interfere with the signal. Another complexity of mixed potential devices is that the catalytic reaction between NO and the sensing electrode consumes oxygen, resulting in a negative relative EMF, while the reduction of NO2 generates a positive EMF through the liberation of O2 causing inaccurate measurement of total NOX concentration.
A number of strategies have been proposed to overcome these limitations. Protective zeolite coatings have been used, which allow gas molecules of only a particular size to pass through to the sensing element, barring the combustion products, hydrocarbons and particulates from affecting the measurement. Alternatively, selective sensing electrode materials may be employed which favor only the oxidation or reduction reaction (such as LaCoO3, which has been identified to be responsive to NO2 but not NO) allowing arrays of mixed potential sensors to be used to determine the NO and NO2 concentration. Similarly, a non-selective sensing electrode can be biased at different voltages to produce an array of sensors which can be simultaneously solved to determine NO and NO2 concentration.
A fundamental concern in the development of mixed potential sensors is that the sensing electrode microstructure controls the non-equilibrium oxygen partial pressure and the kinetics that generate the mixed-potential response. It has also been suggested that microstructure control through the development of multi-component nanocomposite electrodes may allow development of sufficiently responsive and stable electrode materials, but at this time, such devices have not been demonstrated.
Amperometric designs measure the current resulting from a constant applied voltage on an electrochemical cell. A number of amperometric sensor designs have been reported in the literature. Electrolytes of these designs are limited to NASICON, YSZ, and lanthanum gallate electrolytes, operating at temperatures ranging from below 200° C. for NASICON to above 500° C. for the YSZ and lanthanum gallate electrolytes.
Amperometric designs as reported in the literature have commercial viability, as will be discussed below. However, they must overcome the limited current that can be achieved by conventional approaches. The devices disclosed in the literature rely upon the catalytic decomposition of NOX to provide the detected current under the imposed voltage, as shown by the following equations:the reduction of NO2 to NO: NO2→+½O2+NO, and/or  (3)the reduction of NO to N2 and O2: NO→½N2+½O2.  (4)Due to the very low concentrations of NOX anticipated in the applications, the signals achieved by these devices are extremely low, limiting the resolution, accuracy, and detection threshold of these sensors. For tailpipe emissions monitoring of NOX in diesel vehicles, accurate detection of low ppm concentrations of NOX is essential to meeting emissions regulations. Additionally, these low signals require additional shielding to protect from electromagnetic interference.
Impedance-based sensors are the third class of electrochemical devices that have been proposed for NOx sensing applications. In these devices, an oscillating voltage is applied to the sensing electrodes, and the current generated by the voltage is measured. By tailoring the frequency of the voltage oscillations, the response can be selected to correlate with specific non-ohmic contributions to the device resistance. In this approach, the divergent responses of NO and NO2 in mixed potential mode are not observed; instead, signals of the same sign and magnitude are observed. However, these devices are the earliest in development and experience interference from both CO2 and H2O, which will always be present in exhaust streams. Finally, even under simplified operating conditions, impedance-based sensors will require more complex signal processing than mixed potential or amperometric sensors.
Several of the above sensor design approaches have been described in the technical and patent literature. One such device is a multi-chamber potentiometric device, which uses a multi-stage reaction approach to condition the exhaust stream for NOX detection. See for examples: U.S. Pat. No. 5,861,092; U.S. Pat. No. 5,897,759; U.S. Pat. No. 6,126,902; U.S. Pat. No. 6,143,165; U.S. Pat. No. 6,274,016; and U.S. Pat. No. 6,303,011. In an initial reaction chamber, oxygen from an external air stream is pumped into the measurement chamber to oxidize all residual hydrocarbons and carbon monoxide, and convert the NO to NO2. The resultant test stream is then exposed to a mixed potential sensing and reference electrode set. The resulting potential is measured to determine NOX concentration. Given the delay for the required processing of the sample gas, the response time of the sensor is anticipated to be too long (several seconds) for use in vehicle applications.
A second mixed potential sensor using yttria-stabilized zirconia (YSZ) with a zeolite-modified electrode, has been studied for NOX detection. See for examples: U.S. Pat. No. 6,764,591; U.S. Pat. No. 6,843,900; and U.S. Pat. No. 7,217,355. This device only works well at high temperatures, is very sensitive to changes in temperature, and has response times of two seconds or more. Due to the slow response times, this technology has not been employed for mobile applications.
The most prominent sensor type for detecting NOX is an amperometric device relying upon multiple oxygen ion pumps, developed and patented by NGK Insulators in Japan. See for example: U.S. Pat. No. 4,770,760 and U.S. Pat. No. 5,763,763. In this technology, considered by engine manufacturers to be the principal viable commercial NOX sensor, all the molecular oxygen in the exhaust gas stream is electrochemically pumped from the exhaust gas sample, before the remaining NOX can be reduced to N2 and O2 by a catalytic electrode material (typically a Pt/Rh alloy) and the resulting oxygen ionic current measured. These sensors are relatively slow, complex, costly, and cannot sense the low NOX concentrations needed by the diesel engine industry. Additionally, they exhibit a strong cross-sensitivity to ammonia, causing erroneous NOX measurements in ammonia-containing gas environments. To effectively monitor NOX breakthrough in either selective catalytic reduction or lean NOX trap systems, resolution of at least 5 ppm and preferably 3 ppm is needed compared to the 10 ppm accuracy of the NGK sensor.
In other research (see for example G. Reinhardt, et al., Ionics 1, 32-39 (1995)), NO is reported to assist in the electrochemical reduction of oxygen, forming the basis of an amperometric sensor. Because of the electrode and electrolyte materials used, however, the demonstrated cell required a minimum operating temperature of 600° C. At these higher temperatures, O2 and CO2 adsorption are thermodynamically favored over NOX adsorption. See: P. Broqvist, et al., Journal of Physical Chemistry B, 109:9613-9621 (2005). Consequently, Reinhardt and his co-workers did not demonstrate NOX sensitivity in the presence of CO2 or water or at low NOX concentrations, and only demonstrated detection of NOX at high temperatures in simplified gas atmospheres. For operation in diesel engine exhaust systems, the ability to detect ppm levels of NOX in the presence of CO2 and H2O is essential, making this approach impractical for use in these applications.
Accordingly, a need exists for improved sensors for accurately detecting NOX or other target gas species.